The mixed potential chemical phenomenon used in the present invention is well known, and is the electrical potential that is generated or set up by two chemical reactions.
It is also known that the thickness of a magnetic recording layer must be closely controlled. This is due to the fact that the magnetic recording phenomena is dependent upon the thickness of this layer. Forming a magnetic recording layer by the use of a plating process is also known.
Electroless plating is the controlled autocatalytic deposition of a continuous metal film by the interaction of a metal salt and a chemical reducing agent in solution. In this plating process the electrons that are used for reduction are supplied by the bath's chemical reducing agent. As a result, electroless plating solutions are not thermodynamically stable because the reducing agent and the metal salt are always present and ready to react.
After an article to be plated is placed in the plating bath, electroless deposition usually initiates at a later time. The time at which plating onset occurs is variable, and is a function of a number of plating parameters, such as the temperature of the plating bath, the composition of the bath and the characteristics of the surface being plated. While some of these parameters, such as temperature and composition, can be held reasonably constant, the surface characteristics of the article being plated are unpredictable, and have been found to be highly dependent upon prior handling of the article, also known as the prior history of the article.
For example, we have found that the start of electroless plating can occur from zero to 45 seconds after the article to be plated is placed in the plating bath. In an extreme case, plating onset may occur as long as 45 minutes after the article has been placed in the plating bath.
If the plated layer's maximum thickness is not critical, the article can be plated long enough to produce a minimum layer thickness, without regard to how much longer the article is plated. However, this is a costly manufacturing process.
When magnetic recording media is manufactured, the magnetic layer must be plated to a closely controlled thickness.
Due to the inherent properties of the electroless plating process, it is difficult to control the thickness of the plated layer, especially during continuous manufacture involving the sequential plating of a number of articles.
Others have addressed problems of this type. For example, in U.S. Pat. No. 4,556,845 an eddy current probe is subjected to electroless plating along with the article to be plated. As the thickness of an electrically conductive plated layer increases on the probe's sensing surface, the eddy currents flowing in this plated layer increase. This increase in eddy current flow is used as an indirect measure of the thickness of the layer being plated on the article.
U.S. Pat. No. 4,477,484 teaches another means of indirect measurement in an electroless plating process. In this patent, the article to be plated is plated along with a "test coupon" that is structurally very much like the article to be plated. This coupon includes electrical circuit conductors that are separated by an insulating area that will be plated. As plating initiates, the decreasing resistance that is measured across these conductors is used as an indirect measure of the initiation, progression and quality of the electroless plating that is occurring at the article to be plated.